This invention relates to apparatus for either (1) continuously producing photovoltaic devices on a substrate by depositing successive semiconductor layers in each of at least two adjacent deposition chambers through which the substrate continually travels, or (2) the batch processing production of photovoltaic devices by depositing successive semiconductor layers in each of at least two unconnected deposition chambers into which the substrate is successively transported. Since the composition of the amorphous semiconductor layers is dependent upon the particular process gases introduced into each of the deposition chambers, even small amounts of impurities or contaminants in the semiconductor layers deleteriously effects the efficiencies of photovoltaic devices produced. Therefore, process gases introduced into the deposition chambers, as well as the cleanliness of the deposition chamber itself, must be carefully controlled. To that end, the deposition chamber is sealed to the atmosphere, pumped to low pressures, heated to high temperatures and flushed with a gas such as hydrogen or argon prior to initiation of the glow discharge deposition process.
In glow discharge deposition techniques currently employed, the process gases are introduced at spaced intervals along one of the sides of the deposition cathode. The process gases are drawn by a vacuum pump across the deposition surface of a substrate where an r.f. powered cathode or a microwave generator creates an electromagnetic field in the region defined between the deposition cathode or microwave generator and the substrate (hereinafter referred to as the "plasma region"). The process gases, upon entering the electromagnetic field are disassociated into a plasma adapted to be deposited onto the exposed surface of the substrate.
However, it has now been determined that the semiconductor material produced adjacent the upstream section of the substrate, that section of the substrate first contacted as the process gases flowing across the deposition surface thereof, exhibits electrically inferior characteristics as compared to the semiconductor material produced over the remainder of the downstream deposition surface of the substrate. The electrically inferior characteristics of the upstream semiconductor material can be attributed, inter alia, to (1) impurities in the process gases initially entering the plasma region of the deposition chamber, (2) contamination from the ambient conditions existing in said deposition chamber when the process gases first contact the energized electromagnetic field, and (3) the changing chemical combinations and bonding formations which are formed as the process gases move across the electromagnetic field.
More particularly, despite efforts to procure "pure" process gases, at least trace amounts of impurities are present. In prior glow discharge deposition apparatus, these impurities were deposited as the process gases contacted the electromagnetic field at the upstream side of the substrate. Further, despite pumping and cleansing efforts, contaminants would outgas from the walls of the deposition chamber when the deposition cathode or microwave generator was powered to create the electromagnetic field. These impurities and contaminants would be deposited on the upstream side of the substrate, thereby contributing to the electrically inferior upstream semiconductor material.
It has also been found that the composition of the semiconductor film deposited onto the substrate in such prior deposition apparatus varies with the length of time the process gases are subject to the effects of the electromagnetic field. In other words, the species and compounds formed when the process gases initially come into contact with and are disassociated by the electromagnetic field vary from the species and compounds deposited onto the substrate at a more downstream location. Although, the precise physical and chemical properties of the species and compounds deposited at the downstream location are currently being investigated and have not as yet been fully identified, it is apparent that they provide superior electrical responses (as compared to the responses of the material deposited at the upstream location).
Whether those improved electrical responses are due to the removal of trace impurities from the process gases, the removal of contaminants outgassed from the walls of the deposition chamber, the formation and breakdown of species and compounds, or a combination of the foregoing, it is clear that the properties exhibited by the material deposited onto the substrate is dependent on the length of time spent in the presence of an electromagnetic field. In other words, the overall electrical properties of semiconductor devices produced from semiconductor layers deposited onto a substrate are superior at the downstream segment of the layered substrate.
Accordingly, it is the principle object of the upstream cathode system of the present invention to create an electromagnetic field upstream of the deposition cathode or microwave generator for (1) collecting impurities from the process gases and contaminants from the walls of the deposition chamber and/or (2) subjecting the process gases to a predeposition electromagnetic field prior to their introduction to the deposition electromagnetic field. In this manner, an improved semiconductor film is deposited onto the substrate, said film being of substantially uniform and homogeneous composition across the surface of the substrate and exhibiting improved photovoltaic characteristics.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas and which can be doped to form p-type and n-type materials for the production of p-i-n type devices which are, in operation, substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous silicon alloys by glow discharge techniques that have (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) provide high quality electronic properties. This technique is fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980 and be vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, it is believed that fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by smaller band gap materials to absorb the light passed through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltages may be added, thereby making the greatest use of light energy passing through the cells.
It is of obvious commercial importance to be able to mass produce photovoltaic devices by a continuous process. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, filed May 19, 1980 for A Method of Making P-Doped Silicon Films and Devices Made Therefrom, now U.S. Pat. No. 4,400,409; Ser. No. 240,493, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Materials, now U.S. Pat. No. 4,410,558; Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Chamber Deposition and Isolation System and Method, now U.S. Pat. No. 4,438,723; and Ser. No. 359,825, filed Mar. 19, 1982 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells, now U.S. Pat. No. 4,492,181. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material. In making a solar cell of p-i-n type configuration, the first chamber is dedicated for depositing a p-type amorphous semiconductor layer. The second chamber is dedicated for depositing an intrinsic amorphous semiconductor layer and the third chamber is dedicated for depositing an n-type amorphous semiconductor layer.
Whereas, for purposes of mass production, the succession of depositon chambers described hereinabove, is most advantageously employed, a batch processing system may also be used. In such a batch processing system the amorphous semiconductor allow layers can also be deposited in multiple layers over large area substrates to form photovoltaic devices. Batch processing techniques for producing p-i-n type solar cells may proceed in either of two possible manners: (1) a plurality of interlocked deposition chambers are provided wherein a first chamber deposits a p-type semiconductor layer; a second chamber deposits an intrinsic semiconductor layer; and a third chamber deposits an n-type semiconductor layer; or (2) a single deposition chamber is provided which is flushed after the deposition of each layer. In either case, the batch process techniques are accomplished on individual substrate plates in an intermittent mode of operation.
While both systems, batch and continuous, have their own set of operating problems, they both must be kept free of contaminants, which, if deposited with the semiconductor layers onto the deposition surface of the substrate, would harm if not destroy the efficiency and operation of photovoltaic devices produced therefrom. Accordingly, each system must be careful to control the interior environment of its deposition chambers to prevent the influx of contaminants from external sources. After being exposed to the environment, the chambers are pumped, heated and cleansed in an attempt to remove contaminants such as water vapor from the chamber walls. Further, only the purest process gases are purchased for introduction into the chamber and subsequent deposition onto the substrate surface as semiconductor layers. And finally, both systems produce said semiconductor layers by employing very similar operating parameters such as r.f. or microwave power, pressure, process gas mixture, flow rate, temperature, etc.
It should therefore be obvious to those ordinarily skilled in the art that the upstream cathode system of the present invention is equally well-suited for use with batch processing and continuous production apparatus. With both sets of apparatus, it serves the identical function of creating an electromagnetic field upstream of the deposition cathode for (1) collecting impurities from the process gases and contaminants from the walls of the deposition chamber, and (2) initiating the disassociation of process gases into electrically superior species which, when deposited onto the substrate, are of substantially homogeneous chemical composition.
These and other objects and advantages of the present invention will become clear from the drawings, the claims and the description of the invention which follow.